Robert Hanssen was a member of the FBI that served as a Russian and Soviet Spy. Charged with 15 life sentences, his acts are believed to have caused the biggest intelligence disaster of all time.

Hanssen was arrested on February 18, 2001 at Foxstone Park[2] near his home in Vienna, Virginia, and was charged with selling US secrets to the USSR and subsequently Russia for more than US$1.4 million in cash and diamonds over a 22-year period.[3] On July 6, 2001, he pleaded guilty to 15 counts of espionage in the United States District Court for the Eastern District of Virginia.[4][5] He was then sentenced to 15 life terms without the possibility of parole. His activities have been described by the US Department of Justice's Commission for the Review of FBI Security Programs as "possibly the worst intelligence disaster in U.S. history."[6]
@Curionic

While bored during his work with the Manhattan Project Richard Feynman would amuse himself by picking the locks of his colleagues confidential file cabinets and placing prank notes, his colleagues believed a spy had infiltrated the project.